1. Field of the Invention
This invention relates to a watch case of the kind comprising a middle cum glass unit having a central transparent portion and a peripheral side portion together defining an inner cavity for accommodating a movement, a back that closes off the cavity, and assembly means for securing the back to the unit.
The invention is more particularly, but not exclusively, concerned with a water resistant case in which the middle cum glass unit is an integral component made of mineral glass, and the back is made of metal.
2. Prior Art
Watch cases of this kind are well known. Yet, despite their simplicity, they have never been used to any great extent because the assembly of the two parts has always given rise to a delicate problem that has never satisfactorily been resolved.
For instance, German Patent Specification No. 3120799 describes a watch case in which the middle cum glass unit, made of pressed glass, and the back, made of metal, are assembled by means of a snapping arrangement acting directly between these two parts. Clearly, such an arrangement is likely to damage or even break the part made of glass when opening and closing the case.
An assembly arrangement not likely to cause such damage is used in the watch case described in Swiss patent specification No. 554014. The middle cum glass unit of this case is made of sintered corundum, a hard and brittle material, and the back is made of steel. The two parts are here assembled with screws extending through holes in the back and driven into metal plugs secured in holes in the middle cum glass unit. Whereas this assembly arrangement does help to avoid high local pressures, it is however expensive to put into practice as it involves having to drill holes in a material that is difficult to machine and having to fit the plugs.